Revert "remount: Remove the "backing" parameter to fs_mgr_overlayfs_setup."

This reverts commit b20aa434a38e7604c42536a16ac637bfc45a2d15.

Bug: 146403375
Reason for revert: broken build 6078429 on git_master on walleye-user

Change-Id: Idc3b1c0ad5228d63298ce84811624b23d0cea434
diff --git a/set-verity-state/set-verity-state.cpp b/set-verity-state/set-verity-state.cpp
index 6f2b6b1..0a26aba 100644
--- a/set-verity-state/set-verity-state.cpp
+++ b/set-verity-state/set-verity-state.cpp
@@ -101,7 +101,7 @@
   auto change = false;
   errno = 0;
   if (enable ? fs_mgr_overlayfs_teardown(mount_point, &change)
-             : fs_mgr_overlayfs_setup(mount_point, &change)) {
+             : fs_mgr_overlayfs_setup(nullptr, mount_point, &change)) {
     if (change) {
       printf("%s overlayfs for %s\n", enable ? "disabling" : "using", mount_point);
     }
@@ -132,7 +132,7 @@
   auto change = false;
   errno = 0;
   if (enable ? fs_mgr_overlayfs_teardown(nullptr, &change)
-             : fs_mgr_overlayfs_setup(nullptr, &change)) {
+             : fs_mgr_overlayfs_setup(nullptr, nullptr, &change)) {
     if (change) {
       printf("%s overlayfs\n", enable ? "disabling" : "using");
     }